Sport utility vehicles (SUVs) and pickup trucks have grown in popularity among consumers in North America. However, such vehicles are usually more prone to rollover accidents than cars. This is mostly attributed to the higher center of gravity for SUVs and trucks as compared to cars. Even SUVs with independent suspension systems and roll stability control systems may still have a higher tendency to roll over than most cars.
According to statistics from the year 2000, 62% of all SUV deaths occurred in rollovers, which is nearly three times the rate for cars (22%). Some government tests indicate that even the most stable SUV is more likely to rollover than the least stable car. National Highway Traffic Safety Administration (NHTSA) statistics from 2001 estimated that 55% of occupant fatalities in light, single-vehicle crashes involved rollover. Furthermore, in 2001, NHTSA estimated that 60% of the fatalities in vans, 63% of fatalities in pickup trucks, and 78% of fatalities in SUVs were caused by rollover. According to statistics from the year 2002, fatalities in rollover crashes involving SUVs and pickup trucks accounted for 53% of the increase in traffic deaths. In 2002, about 10,626 people died in rollover crashes in the US, up 4.9% from about 10,130 in 2001.
Some rollovers are caused by a vehicle colliding with a curb or abutment during a severe turn or during a lateral slide, which is often referred to as a trip rollover. Even a low profile sports car may rollover when colliding with a trip mechanism. Statistics show that over 90% of trip rollovers are caused by a loss of control of the vehicle. Thus, a need exists to improve vehicle stability during severe cornering or emergency maneuvers.
Some rollovers occur when a driver attempts to avoid a collision with an object (e.g., another vehicle, a person, an animal, etc.) in the road. When a driver swerves to one side (e.g., right) to avoid an object and then attempts to regain control of the vehicle and avoid going off the road by swerving in the opposite direction (e.g., left), this maneuver may cause a vehicle to rollover as well (even when no trip mechanism is encountered). During such maneuvers where the vehicle's weight is shifted from one side to another, as the vehicle suddenly turns one direction (e.g., right) and then immediately turns to back in an opposite direction (e.g., left), the vehicle's suspension springs may contribute to initiating a rollover. This happens because the suspension springs have potential energy mechanically stored as a result of being compressed by the weight of the vehicle.
Even at level straight condition, the weight of the vehicle partially compresses the springs to counteract this weight. This is dramatically demonstrated by a person lifting up on a fender of a 6,500-pound vehicle and being able to move one side of the vehicle upward with ease. When the vehicle's weight is transferred to one side (e.g., right), the spring on that side may be further compressed due to the lateral acceleration of the vehicle and the weight shift toward one side. As the vehicle tilts from one side to another side, as in a right-left maneuver for example, the once compressed spring (during right turning) will push up on the inside of the vehicle (during the immediately subsequent left turning). This pushing up on the vehicle's weight is combined with the lateral forces acting on the vehicle due to the turning motion. This energy stored in the spring can propel one side of the vehicle upward with very little release of pressure on the spring. The vehicle tilt movement caused by the inside spring releasing its stored energy creates rotational momentum that is then added to by the lateral or centrifugal forces created by the turning motion of the vehicle and by the forward momentum from the vehicle's forward movement.
In a severe turn, the suspension system lets the centrifugal force of the turn lower the vehicle on the outside of the turn while at the same time raising the vehicle on the inside of the turn. The upward force applied to the sprung portion of the vehicle by the springs on the inside of the turn is by far the most significant controllable force contributing to loss of control of a vehicle. Thus, the tilt movement initiated by the stored energy in the inside spring may create the momentum needed to initiate a rollover, which the lateral forces of the turning and the forward momentum of the vehicle may bring to fruition. As the vehicle is rotated by this action, it quickly takes less and less pounds of centrifugal force to progress to the next succeeding degree of vehicle rotation. The vehicle in less than one second can be put into a precarious position that can cause the driver to panic as he feels his inability to control the vehicle. This can quickly cause the driver to lose the ability to avoid other vehicles as well as curbs or abutments that can cause a rollover. Hence, a need exists to improve and/or control the stability of vehicles during such severe turning maneuvers. Such improvements may save thousands of lives each year and reduce the number of accidents thereby saving millions of dollars to drivers and insurance companies.